The clouds are a blessing. it’s 97 degrees in the shade, and the air is wet as kelp. For six miles the trail has wound in and out of the Costa Rican rainforest like leather stitching—here a narrow footpath under towering beach almond trees, there a black-sand trudge along a mile of rocky

coast. Twice we’ve forded waist-deep rivers barefoot, crossing between low surf and mist-shrouded forest, watching for crocodiles and bull sharks with our shoes in our hands. We’ve cooled our heads in streams pouring onto the beach from 30-foot cliffs, and clambered over rusting shipwrecks and the graves of forgotten gold miners. We’re now six hours into a 12-mile hike, and ahead the shore of the Osa Peninsula curves into the distance, green forest spilling to the sand, surf gleaming like the edge of a scimitar.

The trail jogs back toward the forest, and there my eyes fall on prints in the sand. Kneeling down, I place a hand beside them. The tracks are deer-like, but more heart-shaped, with rounded tips. In spring and summer, endangered jaguars patrol these beaches for the eggs and young of the four species of threatened sea turtles that nest here. Outside of turtle season, the big cats turn to another food source: white-lipped peccaries, pig-like herd animals that roam the forest sniffing out fallen fruit. It’s the first week of April, a bit early for nesting turtles but pretty good timing for a meal of rainforest hog. Our eyes scan a dense curtain of hibiscus and almond trees where the tracks exit the beach.

This is the wildest and least explored corner of Costa Rica. Dangling like a ripe fig from the country’s southern Pacific coast, the Osa Peninsula is nearly half a billion acres, with a scarcity of roads. A few small villages dot the landscape, but more than a hundred miles of beach are uninhabited by humans, protected in federal and private sanctuaries. At the heart of it is Corcovado National Park, 263 roadless square miles that cover nearly a third of the peninsula.

Some travelers charter tiny planes to access the Osa’s remote landing strips, or hop a boat from nearby small villages to trailheads on the beach. I’m paying my Corcovado toll in sweat: I’ve hired Mike Boston, a tropical biologist who leads nature expeditions, to accompany me for a two-day trip to Corcovado’s Sirena Research Station. One of a scant handful of visitor services in the whole park, Sirena is part backwoods park office, part science laboratory, part wilderness hostel, with open-air workspaces and spare dorms squatting under fig trees. There’s a low-roofed shelter for tent campers and a dirt airstrip where black vultures dry their wings during the day and marine toads, heads swollen with poison glands, hunt at dusk. Foot trails fan out from Sirena, offering access to crocodile-rich rivers and dark groves of wild cashew trees. But today many miles remain before I can plop down on the outpost’s wide wooden veranda.

Born in Pakistan to Irish parents, raised in Trinidad and Tobago, and an adopted Costa Rican by dint of a 17-year-long love affair with the Osa, Boston hikes these sunbaked beaches in worn-out Crocs and surplus camouflage fatigues, a leather machete scabbard knotted around his waist. I keep up with him as best I can, but distractions are everywhere. Though the Osa Peninsula contains only three percent of Costa Rica’s landmass, more than half the country’s plant and animal species can be found here.

I cross a small lagoon fringed with marsh, and walk within five feet of a bare-throated tiger heron, a gray-and-orange- barred beauty whose sunset caterwauls sound like a night-hunting feline. Just beyond, a great kiskadee—one of the largest tyrant flycatchers—forages insects from driftwood snagged in the outgoing tide. All told, more than 370 species of birds have been found on the Osa, including neotropical migrants— prothonotary warblers, for example, and Baltimore orioles—that migrate to breeding grounds back home in my native North Carolina.

With every other step there’s another sighting to impede our progress. A troop of squirrel monkeys passes not 10 feet overhead, tiny babies clinging to the mother like opossums. Boston points out black and orange poison dart frogs tucked into the crevices of fallen logs. Once he steps a few feet off the

trail and bends a tall heliconia plant toward me. Stacked inside a furled leaf like peas in a pod are tiny, two-inch-long Spix’s disk-winged bats. Every few nights, as the leaf tube grows wider, the bats must seek out another suitably sized roosting plant. I hold the leaf like a vase, and feel the bats trembling in my hand.

Now, 50 yards ahead of me, Boston stands on the edge of the beach, motioning my way. He holds a finger to his lips, and we crouch low, threading shoulders through swinging vines and heavy stands of cane. When he nods toward the forest ahead, it takes me a few moments to make sense of the brush and shadows. Then a mound of leathery hide rises from the mud. With my binoculars I make out white-tipped ears and a short, prehensile snout, smooth and elephantine. I can’t believe our luck. This is the endangered Baird’s tapir, a giant creature that has changed very little in the past 35 million years.

Tapirs can weigh 850 pounds and are built like barrels, low to the ground, with short, powerful legs and splayed toes to keep them from sinking into the muck. Once abundant from southeast Mexico to Panama, the Baird’s tapir has been pushed by logging and hunting into mere pockets of its former range. Although these animals, most closely related to horses and rhinos, aren’t known for a pugnacious attitude, we keep our mouths shut and our movements slow. For five full minutes the tapir munches on what seems to be a 50/50 mix of mud and leaves. Then the big fel- low goes quiet. Through my lenses, all I can see is a patchy mound of mud, rising and falling. It’s naptime for the largest land mammal in Central America.

We leave him to it, and head down the trail toward the Sirena station, where we will bunk for the night in a low-roofed shelter among fellow backpackers from Europe, local ticos on vacation, and the occasional resort guest with a serious hankering for tropical birds.

After wilderness trekking for two days with Boston, I spend the balance of my trip base camping at Bosque del Cabo, a nature lodge overlooking the soaring, 500-foot-tall cliffs of Cabo Matapalo, on the southernmost tip of the Osa (see “Making the Trip,” below).

When Florida surf bum Phil Spier stumbled onto Bosque del Cabo’s cliffside tract in 1979, it was hardly an oasis. Nearly always barefoot, with tousled, graying hair, Spier remembers rough, eroded cattle pastures sprawling almost to the cliffs. Native wildlife was scarce. “When I first got here,” he recalls, “every little farm had eight hunting dogs lying around. Nothing that lived on the ground lived for very long.” Macaws were peddled on the streets of Puerto Jiménez. Monkeys were sold as bushmeat.

That’s no longer the case. Today Bosque del Cabo encompasses about 750 acres of forest filled with giant corozo palms and sun-loving cecropia trees. On a one-hour-long hike from the lodge to the beach, I wander into a flock of

great curassow, tall pheasant-like birds whose skull crests of exuberant feathers look like curly shavings of dark chocolate. Gray ground birds called great tinamous sing throughout the forest, their ghostlike, warbling calls ringing from all directions. Cat prints stipple the trail, but I can’t tell what they might be, since five of Costa Rica’s six wild felines—jaguar, puma, ocelot, margay, and jaguarundi—have been seen at or around the lodge. The squawks of scarlet

macaws lead me down to a beach where surfers ride long swells into a curving, undisturbed lagoon.

Protected private lands such as Bosque del Cabo draw a bounty of wildlife from Corcovado and other preserves, but it’s not a simple exchange. For all their grandeur and importance, the peninsula’s protected lands frequently share borders with palm oil plantations, cattle ranches, and subsistence farms. Parks like Corcovado, says Manuel Ramirez, are “islands of genetics for wildlife and plants, and it is vital to get those genetics beyond their borders, past the pasturelands, and into the rest of the peninsula.”

Ramirez is the cofounder and director of Osa Conservation, a nonprofit that runs a pair of research and education stations on the peninsula and manages the Osa National Wildlife Refuge, part of Costa Rica’s innovative private wildlife reserve system. Now encompassing 5,600 acres, the reserve seeks to protect a contiguous biological corridor between Corcovado National Park and Cabo Matapalo, a dozen miles to the east. Participating landowners, including Bosque del Cabo, are exempt from federal property taxes and receive protection from illegal timber cutting and squatters.

The scale of threats to the Osa Peninsula’s wildlife and habitat go beyond a few unauthorized loggers, however.

Escalating gold prices have prompted a rash of illegal mining, particularly inside Corcovado, where Ramirez says there are several hundred miners sifting for gold in rainforest streams. In recent years activists have halted a proposal for an industrial yellowfin tuna farm that would have operated less than a mile offshore. Rising global demand for palm oil has fueled a 17 percent increase in southern Costa Rica acreage planted in palm oil plantations between 2007 and 2011. Even tourism run amok is a concern. As word gets out about the wonders found on the grounds of the region’s small-scale ecolodges, there’s a push to increase visitor numbers. Costa Rica plans an international airport and a large-scale marina here. If those plans are realized, hotels will likely crop up in outposts like Puerto Jiménez, where a single landing strip now serves as the Osa’s primary airport.

“Jet skis on the beach, Jeeps in the rainforest—this is what people are worried about,” Ramirez explains. During the past few years more and more locals have opened small tourism businesses, from families providing two-hour horseback tours of the rainforest and beach to campesinos opening their properties to wildlife hikes. The Osa model of conservation will work, Ramirez says, only if the scale of tourism development remains compatible with the needs of wildlife.

Deep in the gloom of the late-afternoon rainforest, Aida Bustamante passes her hand in front of a motion-sensor camera cabled to a stout trailside tree. Nothing happens. She waves again to break the invisible beam and trigger the shutter. No response. Ten seconds after the third pass, the camera’s flash fires. “This is the problem,” she sighs. “Humidity is so hard on the cameras. It takes much work just to keep them working.” Which means Bustamante has plenty of work to do. We’re hiking along the Titi Trail, which winds through thick balsa and cecropia trees on Bosque del Cabo’s grounds. A wildlife biologist of Costa Rican and Panamanian descent, Bustamante founded the nonprofit organization Yaguará in 2006 to support an exten- sive motion-sensor camera project designed to monitor popu- lations of jaguars, other wild felines, and their prey. Currently, Bustamante oversees about 75 trail cameras scattered from Bosque del Cabo’s square mile of rainforest all the way to La Leona, on the eastern border of Corcovado National Park, and

an additional 80 or so on the northern side of the park. All week long I’ve been watching for jaguars, the third largest cat in the world and the species that visitors here most want to see. Although they’ve been pushed out of 40 percent of their historical range, jaguars can still be found in 19 countries, from Argentina to Mexico. A few sightings have occurred in the United States, but there is no evidence of a breeding U.S. population in the past half-century. Despite the cat’s enormous range, DNA analysis shows there are no subspecies, making Panthera onca the only large, wide-ranging carnivore with such genetic continuity throughout its entire territory. Conservationists are now pushing for a “Jaguar Corridor” that links protected habitats from northern Mexico to Argentina.

Thirty years ago, says Eduardo Carrillo, a wildlife biologist at the National University of Costa Rica, Corcovado National Park had the highest density of jaguars known in Central America. For 23 years Carrillo has monitored the park’s population of jaguars, white-lipped peccaries, and 15 other species, and he has overseen camera-trap studies in Corcovado since 2003. In 1997, he estimates, 90 to 135 jaguars roamed Corcovado. That population dropped to between 40 and 50 by 2003, “and it has been decreasing very fast since then,” he says. “Corcovado is a special place, and it could help repopulate other habitats. But there are simply not enough park rangers to enforce the law.”

The primary culprit is the poaching of white-lipped peccaries, the big cat’s primary prey, by illegal gold miners inside the national park. Costa Rica has lost 89 percent of these animals, according to the International Union for Conservation of Nature. Globally, the species is red-listed as “vulnerable,” just one level down from “endangered.” Not only does the poaching suppress outright population numbers, it also drives jaguars out of the park and into the surrounding farmlands. There they become vulnerable to ranchers and subsistence farmers who deem the cats a threat to their small herds of cattle, goats, and domestic pigs. Yaguará attempts to reduce such revenge killings of jaguars by reimbursing ranchers for livestock losses. In the past five years it has paid 28 cattle ranchers for more than 57 livestock killed by big cats. “Most are by pumas,” Bustamante says. “But jaguars are always blamed.”

One afternoon, Bustamante and I check cameras at Osa Conservation’s Piro Biological Station, the Osa National Wildlife Reserve, El Remanso Lodge, and Bosque del Cabo. At El Remanso, mantled howlers, among the largest of the New World monkeys, follow us through the forest, grasping vines and limbs with long prehensile tails and roaring at ear-splitting volume, their deep rumbling calls amplified by a hollow bone near their vocal cords. At the Piro station, tiny pacas, burrowing rodents perhaps two feet long and spotted like deer fawns, freeze still as stones when we pass. Watching them, Bustamante shakes her head. Pacas, which are still hunted for meat, can bring more than $40 per pound. “Many of these poachers are not hunting tofeedtheirfamilies,”shesaysquietly.“Theyusethemoneyfor drugs and women, and that is much more sad to me.”

It’s getting dark as we check a last camera trap along Bosque del Cabo’s Titi Trail. Once cleared and slated for development, this parcel is now re-wilding with second-growth heliconias and balsa trees, plus a tangled undergrowth of fruit-bearing vines and shrubs. Back on the twisty gravel road that skirts the property, Bustamante props a laptop computer on the tailgate of her pickup truck, and inserts a memory card taken from one of her trail cameras.

“No question, the farther you get away from ecotourism areas, the worse the problems,” Bustamante explains. “The lodges offer jobs and money, and that makes so big a difference.”

She scrolls through several hundred images, stopping whenever an animal appears in the frame. There are five photos of pumas, three shots of margays. The cameras reveal the astonishing array of life that shares the trails of Bosque del Cabo with human visitors. The din from calling cicadas is so loud now we can barely hear each other speak. We watch the screen, colors from the laptop flickering across Bustamante’s face, greens and golds and browns from the rainforest shadows.

It’s an astonishing array of tropical wildlife, all from one camera, one trail, one tiny dot on the vast landscape. When Bustamante turns the laptop off, I’m lost in the darkness for a few moments, but still I can see the afterimages—coatis, agoutis, red brocket deer, a tapir, a jaguar. And I wonder what will come walking by on this night.

Making the Trip: Costa Rica

Getting there: Most major U.S. carriers fly to Juan Santamaria International Airport in San José, Costa Rica. Two Costa Rican airlines service Puerto Jiménez daily from San José (about $270): Sansa (flysansa.com) and Nature Air (natureair.com). Weight restrictions for luggage vary but are 30 pounds or less per person. The Costa Rica Tourism Board (visitcostarica.com; 866-COSTARICA) is a reliable source of information.

Getting around: Spanish is the official language. At press time, one U.S. dollar was worth about 530 Costa Rican colónes. Renting a car is not necessary to visit much of the Osa. Most ecolodges can arrange ground transportation; some will arrange relatively inexpensive charter flights.

Staying there: Mike Boston’s Osa Aventura (osaaventura.com) offers very personalized guided tours of Corcovado National Park and handles many of the details that can be problematic for tourists, from arranging rooms and meals at the Sirena Research Station to booking custom tours and overnight adventures at farms where birders and wildlife enthusiasts are welcome. Bosque del Cabo offers rainforest tours, a zipline through the canopy, biologist-led hikes, and bewildering numbers of birds. Awarded the “5 Leaf ” designation, the highest in the Costa Rica Tourism Board’s Certification in Sustainable Tourism program, the lodge has 15 cabanas and cabins ($135–$235 per person, per night, including meals, depending on the season), all clustered in a tidy compound on one edge of the lodge’s 750 acres.

For more information: Corcovado National Park is rugged and remote and offers few visitor services. It’s best to plan a trip through an established nature lodge or an outfitter such as Osa Aventura (above). Go to visitcostarica.com.

This story ran in the May-June 2014 issue with the title "It's A Jungle In There."